1. Technical Field
The present disclosure relates to a calibration circuit and a calibration method for a digitally controlled oscillator (DCO), e.g., an oscillator configured to generate a clock signal the frequency of which can be trimmed in a discrete way by means of a digital control signal.
2. Description of the Related Art
High-precision amplifiers, in particular for processing signals in microelectromechanical systems (MEMS), use open-loop oscillators for providing operations of amplification, sampling, or chopping in switched-capacitor circuits, and likewise operations of conversion in the digital domain.
These oscillators, the oscillation frequency of which typically does not exceed a few megahertz, undergo coarse calibration or trimming during fast-test procedures. It is desirable to calibrate the oscillators as fast as possible in order to minimize the testing time. On the other hand, for the testing steps, a perfect synchronism between the signal generated by the oscillator and a reference signal is not required.
A circuit for discrete trimming of a voltage-controlled oscillator (VCO) is known, for example, from T. H. Lin et al., “A 900-MHz 2.5-?mACMOS Frequency Synthesizer With An Automatic SC Tuning Loop,” IEEE JSSC, 2001. However, said document describes a closed-loop calibration technique, which requires a long calibration time (a PLL circuit must be pre-set before it is possible to generate a result that can be used for calibration purposes).
The document by H. I. Lee et al., “A ΔΣ fractional-N frequency synthesizer using a wideband integrated VCO and a fast AFC technique for GSM/GPRS/WCDMA applications,” IEEE JSSC, 2004, describes a circuit for open-loop calibration of an oscillator. However, the calibration time is relatively long, in so far as a plurality of counters are provided, which, in order to supply a usable result, must accumulate a certain number of counts.